I keep needing to look this up as AC season is coming on, so I thought it might be helpful to others as well.

[This message has been edited by buddycraigg (edited 05-26-2009).]

Buddy: Thanks for posting this but I'm not sure if this chart provides accurate info. or perhaps I am not reading it correctly. At about 85*F ambient my R-134A systems read between 150-200PSI on the high side line. This seems to be typicl pressure for an R-134A system. According to this chart at 85*F the R-134A pressure should read 86.4 psi? Not sure how this relates. What is your take on it?

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Here's how I read this: Vapor pressure is the pressure at which a liquid vaporizes at a given temperature. So, looking at the chart, if I have R134a in my system, and the low side pressure is 30.9 (compressor running), then the refrigerant is vaporizing at 36 degrees.

bump,
so i can find it easier

Vapor pressure is the pressure a liquid gives off at a given temperature. Example - Water has a vapor pressure at all temperatures and it increases with temp. At 212 deg water's vapor pressure is about 14.7 psi which is atmospheric pressure so now the water boils

quote Originally posted by Phil: Vapor pressure is the pressure a liquid gives off at a given temperature. Example - Water has a vapor pressure at all temperatures and it increases with temp. At 212 deg water's vapor pressure is about 14.7 psi which is atmospheric pressure so now the water boils

So how does this chart help us set low and high side A/C charging pressures? Actuallly how does this chart help us at all?

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

You're sounding kinda hateful.

I try to charge a system by weight whenever possible.

but this is how is it helpful, at least to me.
say I'm only getting 50degrees out of the vents, but my low pressure is 27psi.
I know my evap is getting cold enough, so i must have a blend door problem, or and air restriction in the fins of the evap.

or do it backwards, I point my thermometer at the accumulator a it's reading 40degrees but the compressor is on steady.
then I know my pressure is too high, either from a blockage at the expansion valve or compressor is weak.

[This message has been edited by buddycraigg (edited 08-04-2009).]

quote Originally posted by buddycraigg: You're sounding kinda hateful. I try to charge a system by weight whenever possible. but this is how is it helpful, at least to me. say I'm only getting 50degrees out of the vents, but my low pressure is 27psi. I know my evap is getting cold enough, so i must have a blend door problem, or and air restriction in the fins of the evap. or do it backwards, I point my thermometer at the accumulator a it's reading 40degrees but the compressor is on steady. then I know my pressure is too high, either from a blockage at the expansion valve or compressor is weak.

Buddy: You've posted lots of good info on this forum and much of it is helpful. My intent on this post is certainly not negative. . I just don't understand how this chart is useful so I asked a quesion. I do all of my own A/C fabrications on swaps and can provide high and low side charging pressure measurements on each relative to ambient temps. This chart doesn't come close to real world A/C system readings so I don't understand how the info can be used. No insult, no hate, just a simple question about the chart so that I can understand why it was posted.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

feel free to post a chart that is more accurate.

I googled "freon vapor pressure r12"
I compared the first 5 charts that i could get, and they all conflicted each other.

EDIT, cause i cant spell google

[This message has been edited by buddycraigg (edited 08-05-2009).]

quote Originally posted by Dennis LaGrua: Buddy: Thanks for posting this but I'm not sure if this chart provides accurate info. or perhaps I am not reading it correctly. At about 85*F ambient my R-134A systems read between 150-200PSI on the high side line. This seems to be typicl pressure for an R-134A system. According to this chart at 85*F the R-134A pressure should read 86.4 psi? Not sure how this relates. What is your take on it?

PT charts SHOULD all show pretty close to the same values. Here is another one for comparison. I didn't check to see how it compared to the one buddy posted.
Also according to his chart it looks like 85F is about 95psi not 86 so I'm not even sure you were reading it right.

http://www.csgnetwork.com/r134apresstempconv.html

The PT chart buddy posted is just like any other PT chart, for high side pressures you have to add 30 degrees to the ambient temperature to get your saturation temp and that is what you will look for on the chart. So if its 85 out, add 30 and look up 115 degrees on the chart which is around 158 PSI. That should get you pretty close.

Our club had a tech session about A/C systems given by a super duper A/C guy and he said you can't correctly charge an A/C system just using gauges. The only proper way is by weight.

im

quote Originally posted by buddycraigg: feel free to post a chart that is more accurate. I googled "freon vapor pressure r12" I compared the first 5 charts that i could get, and they all conflicted each other. EDIT, cause i cant spell google

The bottom line is that there is no chart that can provide valid pressure readings for every system. In the case of R-134A conversions, there isn't even any factory Service Manual data available. Pressure readings on R-134A conversions vary highly from car to car, system to system, condition and are highly influenced by ambient temperatures. I charge by monitoring pressures and the A/C outltet temps. If you charge only by volume you can end up with some dangerously high port readings.
On a hot 95**F day the high side port on the Fiero system should read 200-250PSI (A/C on full blast max) and low side about 25-35 psi. This is just a general rule.
Now on cars originally equipped with R-134A systems all of the charging info is in the factory manual.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

i am glad you have a system that works for you.

[This message has been edited by buddycraigg (edited 08-06-2009).]